A typical undercarriage of a highway tractor comprises a suspension system which includes suspension of an axle, such as a rear axle, from an undercarriage component, such as a chassis frame. The suspension system absorbs road-induced impacts as the highway tractor travels along a roadway, thereby contributing to quality of ride and handling.
One type of suspension system is a pneumatic suspension system which has gas springs on right and left sides of an axle for absorbing vertical road forces imposed on the axle through the axle's wheels. A gas spring typically has a single point of attachment to an undercarriage, but it may be coupled with an axle through various mechanical configurations, one example of which is a trailing arm.
A trailing arm has a proximal end having a point of mounting on a chassis frame which allows the trailing arm to swing up and down about an axis which is perpendicular to the length of the tractor, toward and away from the chassis frame. A housing of the axle is fastened to the trailing arm at a location rearward of that axis. An upper end of a gas spring has a point of attachment to the chassis frame. A lower end of the gas spring has a point of attachment either to the axle or to the trailing arm. The gas spring will contract axially as the axle swings toward the chassis frame and will expand axially as the axle swings away from the chassis frame.
Any given suspension system may have additional components or mechanisms that perform specific functions such as aiding in maintenance of proper alignment of the axle to the undercarriage.
Certain pneumatic suspensions may provide control of ride height, meaning that they can set a nominal distance of the axle from the chassis frame when the tractor is parked. That distance may be set at any distance within a range of distances using ride height position sensors to measure ride height. When the tractor is moving, the gas springs still absorb vertical road forces imposed on the axle through the axle's wheels riding along a roadway.
When the medium within a gas spring is air, an on-board air compressor can be operated through a control system to increase the quantity of air in the spring, thereby increasing ride height. Bleeding air from the spring decreases ride height.
The wake of a moving tractor-trailer imposes aerodynamic drag on the tractor-trailer as it travels along a roadway with the drag increasing as travel speed increases. Various known methods and devices are currently used to reduce drag. Some involve installing components which may inhibit tractor or trailer usability or change the shape of a trailer in a way which reduces its usable cargo volume.